The present invention relates to a magnetic head device having, in combination, a read/write head unit having a read/write magnetic core and an erase head unit having an erase magnetic core.
A magnetic device has been put into practical use which device has a rotary magnetic medium and a magnetic head assembly adapted to be moved on the rotating medium in the radial direction of the latter thereby to successively write or read signals in or out of the medium. In this type of device, the magnetic head assembly is driven by a stepping motor, linear motor, hydraulic servomotor or the like. In such driving means, it is not always possible to obtain a constant pitch of movement of the magnetic head assembly so that the position of the magnetic head assembly is often deviated from the aimed position corresponding to the destined track. This inconveniently causes such a problem that the magnetic read/write device cannot stably read the signals in the medium when the signals has been recorded in the medium by another magnetic device, due to the offset of the track.
In order to obviate this problem, a proposal is made in U.S. patent application Ser. No. 387564 filed on Aug. 10, 1973, wherein a read/write head unit having a read/write head gap and an arase head unit having an earse head gap are combined with each other in such a manner that there is accurately erased the edge of a track adjacent to another track in which an information is newly written.
In the magnetic head assembly of the kind described, the track density can be increased by reducing the thicknesses of respective cores. A mere thinning of the cores, however, causes such a problem particularly when the material of the core has a low saturation flux density that the magnetic flux emerges not only from the gap in the core but also from the portions around the gap due to low level of magnetic saturation of the core, although it is desirable that the magnetic flux emerges only from the gap portion of the core.
Various approaches have been made for increasing the coercive force of the medium in order to attain a higher writing density in the magnetic medium. One of such approaches is to use an Mn-Zn ferrite having a high saturation magnetic flux density as the material of the ferrite head. The Mn-Zn ferrite, however, suffers various problems such as inferior mechanical strength and difficulty in the control of atmosphere in high-temperature treatment such as bonding, as compared with Ni-Zn ferrite.
Namely, when the temperature of the Mn-Zn ferrite is raised in the atmosphere containing excessive oxygen, the oxygen is inconveniently included into the ferrite to make the ferrite too rich in oxygen thereby to deteriorate the magnetic characteristics of the ferrite. The deterioration of the magnetic characteristics takes place also when the atmosphere does not contain sufficient oxygen. It is also to be noted that the Mn-Zn ferrite has a greater tendency of reaction with the bonding glass than the Ni-Zn ferrite does.
It may be possible to eliminate the above-described shortcomings by using the Ni-Zn ferrite as the material of only the erase head core in the magnetic head assembly. Such a countermeasure, however, imposes a new problem concerning the coefficient of thermal expansion. Namely, the Mn-Zn ferrite and Ni-Zn ferrite exhibit mean thermal expansion coefficients at temperature ranges between 0.degree. and 300.degree. ranging between, respectively, 110.times.10.sup.-7 and 130.times.10.sup.-7 .DELTA.l/l/deg. and 80.times.10.sup.-7 and 90.times.10.sup.-7 .DELTA.l/l/deg. The difference between these coefficient values exceeds 10.times.10.sup.-7 .DELTA.l/l/deg which is considered as being the upper limit of the difference in this field of art for glass bonding, so that there is caused a fear of occurrence of cracking in a case where the Mn-Zn ferrite and the Ni-Zn ferrite are bonded by means of a glass, and this is the reason why the bonding of these two kinds of ferrites has been made by a resin but not by a glass. The resin used for bonding, however, is much more unstable against change in temperature and humidity than glass. Consequently, the resin bonding often causes a change in distance between layers bonded each other and in bonding strength, resulting in a deterioration in the precision of the magnetic head.